1. Field of the Invention
The present invention relates generally to a fisheye lens having a field angle of 180.degree. and, more particularly, to a fisheye lens that is small of fluctuation in aberration during short-distance focusing as well as being bright.
2. Related Background Art
Originally, the fisheye lens is a special lens which has been employed and developed for the scientific purpose as used for, e.g., all-sky observation photographing, etc. because of having a special projection system for projecting a semi-spherical surface on the plane.
In recent years, however, with the developments of photographs and cameras, the fisheye lens has been increasingly used not for that special projection system but for exhibiting an effect in terms of delineation due to a large negative distortion. When enhancing the effect of delineation by use of the fisheye lens especially in the field of artistic photographs, unlike the scientific-purpose usage in which the optical axis of the lens is directed to the zenith, there are required not only the optical performance for an infinity object point but also an optical performance in which the picture is uniform and excellent from the infinity object point to a short distance object point as needed in the typical wide-angle lens.
Now, supposing that the fisheye lens is simply conceived as a retrofocus type super wide-angle lens having a large distortion, it is required that a compensation thereof be done because of the performance being poor particularly at the short distance.
Seeing the recent techniques, however, for instance, Japanese Patent Application Laid-open No.2-248910 discloses no technique of compensating the fluctuations in aberration during short-distance focusing. Besides, this type of fisheye lenses in recent years has increased in its aperture ratio and has been constructed more compactly. Contrarily, those fisheye lenses have a tendency in which the fluctuation in aberration during short distance focusing rises.
Generally, the fisheye lens has a necessity for causing a large negative distortion. Hence, the basic structure is the retrofocus type having an extremely asymmetric arrangement of refracting power. Originally, in the retrofocus type lens, when focusing on the short distance object with an overall extension, a field curvature outstandingly fluctuates in a positive direction. The short distance performance worsens.
In the case of the fisheye lens, however, the remarkable negative distortion is produced with the extremely asymmetric arrangement of refracting power. Therefore, when focusing on the short distance object with the overall extension, resultantly the field curvature fluctuates strikingly in the negative direction reversely to the typical retrofocus type lens. Consequently, this also undesirably leads to worsening of the short distance performance.
The outstanding short distance fluctuations described above can be seen especially in the fisheye lens having the larger aperture ratio and that is constructed more compactly. It has been therefore desirable to develop the fisheye lens exhibiting a smaller fluctuation in the aberration at the short distance and a higher performance.